1. Field of the Invention
This invention relates to a magnetic field detecting apparatus comprising a multilayer film of the induced ferrimagnetic type or the like having a giant magnetoresistance ratio, especially a magnetic field detecting apparatus comprising a magnetoresistive head capable of reading a small magnetic field change as a large electrical resistance change signal and a method for preventing output drop of such a multilayer film.
2. Prior Art
With recent growing demands for increasing the sensitivity of magnetic sensors and increasing the density of magnetic recording, active research works have been devoted for the development of magnetoresistance effect type sensors (referred to as MR sensors, hereinafter) and magnetoresistance effect type heads (referred to as MR heads, hereinafter) both utilizing a magnetoresistance change. Both MR sensors and MR heads are designed to read out external signals by detecting changes in the electrical resistance of a magnetic field sensor formed of magnetic material. In conjunction with size reduction of recording medium, conventional inductive heads adapted to read information from the recording medium produce an output which lowers with a lowering of the relative speed of the head to the medium, whereas the MR heads have the advantage of producing high outputs in high density magnetic recording since the reproduced output does not depend on the relative speed of the head to the recording medium. Conventional MR heads include a sensor portion generally formed of Ni.sub.0.81 Fe.sub.0.19 (Permalloy) and this Permalloy offers only a magnetoresistance ratio as small as about 2% and is low in sensitivity as the MR head material intended for reading ultrahigh density records of the order of several GBPI.
Recently reported were multilayer films having a significantly higher magnetoresistance ratio than the Permalloy and similar alloy films. These films are designated artificial superlattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked. Among them, the Fe/Cr superlattice was reported to exhibit a magnetoresistance change in excess of 40% at cryogenic temperature (4.2K) (see Phys. Rev. Left., Vol. 61, page 2472, 1988). This artificial superlattice, however, is not commercially applicable as such because the external magnetic field at which a maximum resistance change occurs (that is, operating magnetic field intensity) is as high as ten to several tens of kilooersted (kOe). Additionally, there have been proposed artificial superlattices of Co/Cu or the like, which also require too high operating magnetic field intensity.
Among these giant magnetoresistive films, one artificial superlattice known as the induced ferrimagnetic type can be operated at a relatively low magnetic field intensity (Jpn. J. Appl. Phys., Vol. 59, page 3061, 1990). It includes two magnetic layers of different coercive forces Hc with a non-magnetic layer interleaved therebetween wherein antiparallelism of magnetization of two magnetic layers is induced by an external magnetic field between the two different coercive forces Hc, exhibiting a resistance change of about 10% at a magnetic field of about 100 Oe.
Also Japanese Patent Application Kokai (JP-A) No. 218982/1992 proposes a structure wherein magnetic thin films disposed adjacent each other via a non-magnetic layer have different Hc, each of the layers having a thickness of up to 200 .ANG.. In Japanese Patent Application No. 203562/1991 which was filed prior to the above-identified application, the magnetic layers have a specific squareness ratio. J. Mag. & Mag. Mater., Vol. 99, page 243, 1991 discusses about a magneto-resistance change ratio and a temperature change of resistivity.
However, the MR magnetic head using a giant MR film of the induced ferrimagnetic type experiences a substantial change with time or has the problem that it produces a high output at the initial, but the output gradually drops.
Physical Review B, Vol. 43, page 1297, 1991, proposes a multilayer film allegedly of the spin-valve effect. This multilayer film includes a sandwich of a magnetic layer of NiFe or the like having a pinning layer of Fe-Mn deposited thereon and a magnetic layer with a non-magnetic layer interleaved therebetween such that antiparallel magnetizations of the magnetic layers are induced by an external magnetic field between the magnetic layer's Hc and the magnetic layer-pinning layer exchange interaction energy, thereby providing a large magnetoresistance change. This was also found to experience a large output drop with time like the previous ones.